Resistance 3

The Magnum is available right from the beginning of the game. Its primary fire shoots explosive rounds that can be detonated by pressing . All you need to do is kill 2 enemies at once with once explosion. This must be done 5 times. There is no shortage of places to get this trophy, but the earliest is Haven, Ok: Haven Outskirts. You will be on inside a building with no wall on 2 sides. Within a few seconds, a Chimeran dropship will arrive and drop several Chimera. Simply send a few rounds into the group, then detonate them, which will typically kill most of them.

The trophy can also be done easily against Leapers. In Mt. Pleasant, after agreeing to find Johnathan, you will eventually come to a few rooms full of Leapers. 1 shot and explosion will easily kill several Leapers, provided they are bunched up. Blow up a few of the Leapers at once, then let the rest kill you. Your progress will be retained, and all of the Leapers will respawn, allowing you to continue to working towards this trophy.

The kills towards this trophy carries over throughout any playthroughs and chapter select, so you don't need to worry about missing it. As usual, you can guage your progress from the Extras menu, under the Rewards section.